With the advance of micro-miniaturization in recent years in the manufacture of semiconductor and like devices, and semiconductor chip mounting boards, there has been a demand for higher resolving power in the exposure apparatus that prints the patterns therefor. To satisfy this demand, the wavelength of the light source of the exposure apparatus must be shortened and the NA (i.e., numerical aperture of the projection optical system) must be increased. Nevertheless, if the wavelength is shortened, optical glass that can withstand practical use is limited due to the absorption of light. Furthermore, when it comes to the short wavelengths of ultraviolet rays and X-rays, usable optical glass materials are nonexistent. In this case, it is completely impossible to construct a reduction projection optical system as a dioptric optical system or a catoptric optical system.
A catoptric reduction projection optical system is disclosed in, for example, Japanese Patent Application Kokai No. Hei 9-211332. The projection optical system disclosed therein comprises two sets of optical systems comprising reflective surfaces with a concave-convex-concave configuration, and forms an intermediate image between these two sets of reduction projection systems. The system has a total of six reflective surfaces, and so has a large number of degrees of freedom for correcting aberrations. However, it has the problem of a large loss in the quantity of light reaching the image plane due to the excessive number of reflective surfaces. Furthermore, the imaging performance of the overall projection optical system declines due to aberrations generated by manufacturing errors in each reflective surface. Consequently, the system is difficult to manufacture since the tolerance of each reflective surface must be kept extremely tight. Accordingly, although imaging performance from the standpoint of optical design is excellent, there is a risk that the imaging performance of an actually manufactured system will be insufficient.
U.S. Pat. No. 5,153,898 (the '898 patent) discloses another type of catoptric reduction projection optical system wherein X-rays from a reticle are sequentially reflected by a first concave mirror, a convex mirror and a second concave mirror. In addition, the same patent discloses a constitution that prevents interference between the wafer and the X-rays during scanning by arranging a plane mirror between the reticle and the first concave mirror, between the second concave mirror and the wafer, or between both. In the '898 patent, the wafer interferes with rays in the synchronous scanning of the reticle and wafer, inviting the eclipsing of the rays. Furthermore, since the angle at which the X-rays intersect on the plane mirror is on the order of 45.degree., and since the X-ray reflective mirror is generally constituted by layering a multilayer film, phase shifts arise due to differences in the incident angles, thereby generating aberrations.
U.S. Pat. No. 5,917,879 (the '879 patent) discloses an X-ray lithographic catoptric reduction imaging optical system comprising, objectwise to imagewise, a first concave mirror, a plane mirror, a convex mirror and a second concave mirror arranged coaxially. Each concave mirror and convex mirror are formed as an aspherical surface. The convex mirror is arranged in the pupil plane, and the system is imagewise telecentric. The '879 patent avoids eclipsing of the light beam that occurs when constituting a catoptric optical system with a plurality of mirror systems. However, to do so, the NA on the wafer side is required to be small (0.06). Accordingly, sufficient resolution cannot be obtained.